Advertisement

Advertisement

Extreme morning sickness? You’re less likely to have a boy

Rex/Shutterstock

By Bas den Hond

Women who suffer extreme morning sickness may be less likely to give birth to sons.

Hyperemesis gravidarum (HG) involves severe vomiting – sometimes up to 50 times a day – and its causes are unknown. Kate Middleton suffered from it during both of her pregnancies. Now a study of 1.65 million pregnancies in Sweden has found that less well-educated women are more likely to develop HG, and that women who develop HG are more likely to have daughters.

Advertisement

Using education level as an indicator of someone’s social and financial status, Lena Edlund of Columbia University in New York and her team found that women who left school at 16 were 76 per cent more likely to develop HG than women who went on to attain masters or PhD degrees.

Their analysis also revealed that, regardless of socioeconomic status, women who have HG are less likely to give birth to sons. “Normally, slightly more boys than girls are born, we don’t quite know why that is,” says Edlund. But her team found that 56 per cent of the women in the study with extreme morning sickness who had successful pregnancies gave birth to daughters.

Women with HG have a higher chance of miscarriage, and it is possible that a male fetus is more likely to be lost than a female one. Out of the nearly 18,000 women in the study who had HG, 6000 pregnancies didn’t reach full term. If these women had started out with fetuses matching the normal sex ratio, then we would expect around 4000 of these lost pregnancies to be boys.

Evolutionary strategy

“The team do not know the genders of these miscarried pregnancies. But if more were male, this could back up an old evolutionary idea. First proposed in 1973, the Trivers-Willard hypothesis suggests that when times are good, it is best to have a son; but in tough times, a daughter is the safest bet.

The reasoning behind this theory is that in many species, strong males try to monopolise females, while weaker males don’t stand a chance of passing on their genes. If a mother is in poor health or food is scarce – or perhaps if her socioeconomic status is low – her newborn son might fail to find a partner and pass on her genes to the next generation.

Stress does seem to affect sex ratio in humans. A number of studies have shown that traumatic events, such as the murder of President John F. Kennedy, the 9/11 attacks, and the Troubles in Northern Ireland, are associated with a dip in the number of boys born locally in following months. But the mechanism for how stress affects sex ratios has remained elusive. Edlund thinks HG could be responsible. “Vomiting is like forced fasting, lowering the blood sugar levels. Fasting and dieting have been shown to influence the sex ratio, so it seems plausible that there is a link,” she says.

Emergency signal

That is an interesting finding,” says David Haig, an evolutionary biologist at Harvard University. But he says the study doesn’t prove that HG is the factor skewing the sex ratio, and that female fetuses may be more likely to cause the condition in the first place. There is circumstantial evidence that the hormone human chorionic gonadotropin can cause nausea, and some studies have shown that women carrying female fetuses have higher levels of this hormone. It’s therefore possible that female fetuses could be triggering HG in this way, although Haig says this evidence is from later in pregnancy than when the condition mostly occurs.

Scott Forbes, a biologist at the University of Winnipeg in Canada, thinks rather than acting as a sex-ratio skewing adaptation for stressful times, HG is more likely to be a sign that something is going wrong. “Given that HG is sometimes fatal, it seems unlikely to be adaptive,” he says.